The present invention relates generally to a reflector for use with flash tubes. More particularly, the present invention relates to a low-cost reflector that is made of a conductive plastic material and incorporates an integral arc guide for controlling the position of the arc within the flash tube. The reflector is suited for use in an automated hematology system in which a pulse of light from the flash tube causes fluorescence of dyed, density-separated layers of blood cells in a capillary tube during centrifugation, so that a complete blood count can be obtained by optical imaging techniques while the blood sample is being centrifuged.
As part of a routine physical or diagnostic examination of a patient, it is common for a physician to order a complete blood count for the patient. The patient's blood sample may be collected in one of two ways. In the venous method, a syringe is used to collect a sample of the patient's blood in a test tube containing an anticoagulation agent. A portion of the sample is later transferred to a narrow glass capillary tube, known as a sample tube. The open end of the sample tube is placed in the blood sample in the test tube, and a quantity of blood enters the sample tube by capillary action. In the capillary method, the syringe and test tube are not used and the patient's blood is introduced directly into sample tube from a small incision made in the skin. In either case, the sample tube is then placed in a centrifuge, such as the Model 424740 centrifuge manufactured by Becton Dickinson and Company.
In the centrifuge, the sample tube containing the blood sample is rotated at a desired speed (typically 8,000 to 12,000 rpm) for several minutes. The high speed centrifugation separates the components of the blood by density. Specifically, the blood sample is divided into a layer of red blood cells, a buffy coat region consisting of layers of granulocytes, mixed lymphocytes and monocytes, and a plasma layer. The length of each layer can then be optically measured, either manually or automatically, to obtain a count for each blood component in the blood sample. This is possible because the inner diameter of the sample tube and the packing density of each blood component are known, and hence the volume occupied by each layer and the number of cells contained within it can be calculated based on the measured length of the layer. Exemplary measuring devices that can be used for this purpose include those described in U.S. Pat. Nos. 4,156,570 and 4,558,947, both to Stephen C. Wardlaw, and the QBC.RTM. "AUTOREAD" hematology system manufactured by Becton Dickinson and Company.
Several techniques have been developed for increasing the accuracy with which the various layer thickness in the centrifuged blood sample can be determined. For example, because the buffy coat region is typically small in comparison to the red blood cell and plasma regions, it is desirable to expand the length of the huffy coat region so that more accurate measurements of the layers in that region can be made. As described in U.S. Pat. Nos. 4,027,660, 4,077,396, 4,082,085 and 4,567,754, all to Stephen C. Wardlaw et al., and in U.S. Pat. No. 4,823,624, to Rodolfo R. Rodriguez et al., this can be achieved by inserting a precision-moldedplastic float into the blood sample in the sample tube prior to centrifugation. The float has approximately the same density as the cells in the buffy coat region, and thus becomes suspended in that region after centrifugation. Since the outer diameter of the float is only slightly less than the inner diameter of the sample tube (typically by about 80 .mu.m), the length of the buffy coat region will expand to make up for the significant reduction in the effective diameter of the tube that the buffy coat region can occupy due to the presence of the float. By this method, an expansion of the length of the huffy coat region by a factor between 4 and 20 can be obtained. The cell counts calculated for the components of the buffy coat region will take into account the expansion factor attributable to the float.
Another technique that is used to enhance the accuracy of the layer thickness measurements is the introduction of fluorescent dyes (in the form of dried coatings) into the sample tube. When the blood sample is added to the sample tube, these dyes dissolve into the sample and cause the various blood cell layers to fluoresce at different optical wavelengths when they are excited by a suitable light source. As a result, the boundaries between the layers can be discerned more easily when the layer thickness are measured following centrifugation.
Typically, the centrifugation step and the layer thickness measurement step are carried out at different times and in different devices. That is, the centrifugation operation is first carried out to completion in a centrifuge, and the sample tube is then removed from the centrifuge and placed in a separate reading device so that the blood cell layer thicknesses can be measured. More recently, however, a technique has been developed in which the layer thicknesses are calculated using a dynamic or predictive method while centrifugation is taking place. This is advantageous not only in reducing the total amount of time required for a complete blood count to be obtained, but also in allowing the entire procedure to be carried out in a single device. Apparatus and methods for implementing this technique are disclosed in the aforementioned copending applications of Stephen C. Wardlaw entitled "Assembly for Rapid Measurement of Cell Layers" and "Method for Rapid Measurement of Cell Layers".
In order to allow the centrifugation and layer thickness measurement steps to be carried out simultaneously, it is necessary to "freeze" the image of the sample tube as it rotates at high speed on the centrifuge rotor. This can be accomplished by means of a xenon flash lamp assembly that produces an intense excitation pulse of light energy once per revolution of the centrifuge rotor. The pulse of light excites the dyes in the expanded buffy coat area of the sample tube, causing the dyes to fluoresce with light of known wavelengths. The emitted fluorescent light resulting from the excitation flash is focused by a high-resolution lens onto a linear array of charge-coupled devices (CCDs). The CCD array is located behind a bandpass filter which selects the specific wavelength of emitted light to be imaged onto the CCD array.
The xenon flash lamp assembly is one of two sources that are used to illuminate the sample tube while the centrifuge rotor is in motion. The other source is an array of light-emitting diodes (LEDs) which transmit red light through the sample tube for detection by the CCD array through a second bandpass filter. The purpose of the transmitted light is to locate the beginning and end of the plastic float (and hence the location of the expanded buffy coat area), the bottom of the blood column, and the meniscus at the top of the plasma layer. Further details of the optical reading apparatus may be found in the aforementioned copending application of Michael R. Walters entitled "Inertial Tube Indexer", Ser. No. 09/032,931, in the aforementioned copending application of Bradley S. Thomas et al. entitled "Blood Centrifugation Device with Movable Optical Reader", Ser. No. 09/033,368 and in the aforementioned copending application of Michael A. Kelly et al, Ser. No. 09/033,373 entitled "Disposable Blood Tube Holder."
Although xenon flash tube assemblies have been used for many years in photographic cameras and the like, the use of such an assembly in an automated hematology system of the type described above poses certain problems. One problem arises from the need to focus the light from the xenon flash tube onto the expanded buffy coat area of the sample tube in a precise and consistent manner, so that accurate layer thickness measurements can be obtained. To some extent, this can be achieved by mounting the flash tube adjacent to an elliptical reflector having one of its foci coincident with the flash tube axis and the other focus coincident with the axis of the sample tube. Unfortunately, however, the electrical arc that is formed within the bore of the flash tube does not always remain aligned with the longitudinal axis of the tube, but instead tends to wander radially from the tube axis in a random manner. In the past, this problem has been addressed by running an external wire, known as an arc guide, along the outer envelope of the flash tube in a direction parallel to its longitudinal axis. The wire is held at a known potential (usually ground or reference potential) and has the effect of drawing the arc into alignment with the wire and hence with the tube axis. While the use of an external wire as an arc guide is an effective solution, it requires that the flash tube remain in a precise rotational orientation with respect to the reflector in order to focus the light at the desired point. The envelope of the flash tube is rather fragile and can be broken by hard contact with external structures of the type required to maintain such an orientation.
Another technique that has been used is to provide the arc guide in the form of a thin metal strip. The metal strip is mounted independently of the flash tube with its edge held adjacent to the outer glass envelope of the flash tube. This approach avoids the need to maintain the tube in a precise angular orientation, but it results in a rather complex flash tube assembly that is difficult to mass produce.
Accordingly, a need exists for an improved flash tube assembly in which the arc guide function is implemented in a manner that does not require precise rotational alignment of the flash tube and does not create the risk of damage to the outer glass envelope of the tube. A need also exists for a flash tube assembly which incorporates an arc guide feature but is nonetheless simple and inexpensive to manufacture.